1. Field of the Invention
The present invention pertains to the separation of suspended and dissolved materials from liquids by flotation, using pregenerated foams having characteristics specifically formulated for the material being extracted from the liquid body or steam.
2. Description of the Prior Art
The art of flotation separation by means of gas, usually air, for commercial purposes has evolved since the turn of the century. The other techniques of removing solids from liquids are filtration, centrifugation, sedimentation (settling) with or without flocculation, oil flotation, distillation or evaporation, and the like. Dissolved materials may be separated by distillation, electrolysis, precipitation followed by filtration, immiscible liquid extraction, etc.
Separation of solids from liquids by flotation is extensively practiced in the field of mineral ore beneficiation. In the early practice, it was found that certain minerals could be preferentially wetted by oils, i.e., their surfaces were oleophilic, or hydrophobic, while the other minerals associated therewith were preferentially wetted by water, i.e., hydrophilic or oleophobic. Thus, as shown in FIG. 1, if a heterogeneous ore is reduced to suitable fineness by wet grinding, and the resulting slurry or pulp is mixed with large quantities of oil, the mixture of minerals can be separated into hydrophilic and hydrophobic fractions. As can be seen, oil coats the oleophilic mineral, but no others. The oil coated particles agglomerate into flotable drops, or form films around air bubbles introduced at the bottom of the slurry body by an air jet, the oil foam formed not only being separated more rapidly from the aqueous suspension, but accomplishing even more complete removal of hydrophobic solids while using only a fraction of the oil quantity required for non-aerated flotation. When the oil floats to the liquid surface it can be recovered by decanting, and filtering the oil would yield the desired minerals.
The success of this so-called froth flotation method stimulated considerable research into the nature of froths and froth films, the relationships between the surface of the various minerals and water and frothing agents, the effects of bubble size, total air volume used, liquid/solids ratios, etc. From such work, it was found that frothing agents other than oils could be used, and that the surface of some minerals, not ordinarily susceptible to froth flotation, could be altered by surface active agents, commonly called "collectors" or "promotors", so as to preferentially associate with air or with frothing agents that, in turn, associated with air bubbles. As the physical chemistry of the liquid-solid-gas interactions became more clearly defined, and as the chemical industry made available greater varieties of surfactants, the list of flotable solids became increasingly longer, and today froth flotation is a wide spread, varied and important ore beneficiation technique.
In recent years, the increasing attention focused on water pollution has encouraged researchers to attempt flotation techniques to remove a variety of polluting species from both industrial and domestic waste water, i.e., sewage.
There are important differences between ore beneficiation and water purification by flotation techniques. The concentration of solids in ore slurries is high, often 30 wt. %, whereas waste waters rarely contain as much as 5%, and usually less than 1% solids. Generally only a single inorganic mineral of high specific gravity is removed in ore practice: wastewaters may contain several polluting species, many of organic compositions and specific gravities close to 1.0, often too diverse in properties for removal by a single combination of collector/frothing agent; they may even mutually interfere with each others' extraction. In ore processing ambient conditions such as temperature, pH, liquid and air flow rates, solid/liquid ratios, etc., can be rather closely controlled; this is rarely possible in wastewater processing. The effluent from ore processing is itself a wastewater, carrying gangue and excess treating agents; the objective of wastewater treatment is to obtain as pure an effluent as possible. The hourly flow rates in ore froth flotation are not only controlled to uniformity but are generally of much lower magnitude than for wastewaters. The removal of dissolved materials is often an important aspect of wastewater purification, but almost never in ore processing.
For these and other reasons, the transference of froth flotation technology from the ore beneficiation application to wastewater purification is neither simple nor complete, and the number of successful commercial applications at present is small.
Some classes of polluting species can be collected by flocculants such as alum, soluble silicates, ferric salts, polyelectrolyte polymers, etc. The usual practice is to add a coagulant or flocculant, and then allow the flocs to settle out of the body of water in quiescent holding tanks, reservoirs or lagoons, the settling requiring many hours even days. Since the settling reservoir capacity must accommodate the maximum flow rate of wastewater, the space requirements for this type of processing are high.
As will be seen from FIG. 2, for some types of wastewater, it has been found that the floc can be separated more rapidly by floating it to the surface by aeration with continuous streams of fine air bubbles. The particles are first collected by a suitable agent into flocs. Neutral flocs are mechanically pushed to the surface by small air bubbles from an air source. Charged flocs may form thick films by collecting around air bubbles. Some colloids, for example, soaps, readily form films which are loaded with particles picked up from polluted surfaces or from the wastewater. The liquid volume of such decanted or skimmed flocs thus obtained is 10-40% of the total wastewater input, just as with the settlement method, but the separation is much more rapid. Processes of this type are not true flotation; the air bubbles do not attach to the flocs, but simply push the relatively large floc masses to the surface at a rate much faster than the flocs would settle by conventional sedimentation. In true froth flotation part of the air remains associated with the floc, and this froth is collapsed after the skimming or other removal step. The resulting floc slurry is usually only 5-20% of the original waste water volume.
In applying the froth flotation principles of ore treatment to the treatment of wastewater, the first step is to find a surfactant having a molecular structure with a hydrophilic portion that has a strong affinity for the polluting species sought, and a hydrophobic portion to preferentially associate with air. If no single surfactant can be found, it may be possible to employ one or more additional compounds, sometimes called coupling agents, that attach to and alter the particle surface, causing it to acquire affinity for a flotation agent added subsequently. As can be seen in FIG. 3, when air is bubbled through the aqueous suspension of such coated particles, the hydrophobic portion of the surfactant is attracted thereto, causing the pollutant to become associated with the air bubbles. As each bubble rises through the liquid body, it may collect many particles, and at the surface of the liquid, the coated particles come together to form films, becoming a fairly stable froth.
FIG. 3 discloses the flotation of particles having a natural surface charge. A surfactant is added to the wastewater and attaches to the surface of the particles. The hydrophobic portion of the surfactant is attracted to the air interface of rising air bubbles from the air source. The smaller each air bubble, the fewer coated particles are needed for form a complete, and therefore stable, film.
As the supply of agent-coated particles is depleted, the partial films that form at the air-liquid interface of the bubbles become increasingly susceptible to separation from the air bubbles by the friction of the travel in the liquid and by turbulence. However, since large volumes of air are used, any material dropped by one bubble will be picked by those following, so that given sufficient time and air, the transference of all the polluting particles to the surface will be fairly complete.
There are several limitations to froth flotation practice. The surfactant must have strong enough hydrophobic characteristics to attach and hold the particles to the air bubble. Enough surfactant must be present on the particle surface to achieve this. The amount of surfactant thus depends on the quantity, size and specific gravity of the particles. Since the turbulence normal to both the introduction of air streams and of upward-rising bubbles tends to shear off the particles from the bubbles, the rate and mode of air feed must be closely controlled, and this determines the retention time for the process. Excessive air input also disturbs the froth layer, which can cause falling out of the collected particles, or the excess air may inflate the existing foam bubbles to the destabilization point, producing a similar affect. In general, the foam blankets or layers from froth flotation processes have minimal stability, and mechanical skimming or other methods for removal of the foam must be affected promptly and as gently as practicable.
Relatively few surfactant or flocculation combinations have been found useful for commercial froth flotation application, especially for high volume wastewater treatment, because they are too sensitive to variations in ambient conditions, because the time and air volumes required are uneconomical, because the agents required are too expensive, or for a variety of other reasons.
Various method are used to introduce air into the liquid in aeration and froth flotation practice. Since turbulence disrupts the accumulation of pollutant particles and destabilizes the floating froth layer, causing loss in sinking of the collected material, gas flow rates must be kept relatively slow and such large orifice devices as spargers and open pipes are unsuitable. Accordingly, special ceramic plates having many fine capillary tubes, finely perforated metal plates, beds of glass or ceramic frits and the like are used, with flow rates low enough to permit the continuous air stream to separate into discrete bubbles as it leaves the air channel orifice. In some cases the bubbles are passed through screens to reduce their size. However, according to Grieves, et al, in Adsorbtive Bubble Separation Techniques, edited by Robert Lemlich (Academic Press, 1972), at page 174, "bubble diameter is generally not at the control of the design engineer, varies with time and position, and is difficult to measure experimentally." For most flotation processes, fine bubbles in the order of 50 to 150 microns are preferred, but their attainment in quantities required for treatment of large volume flows of wastewater is difficult.
A method used to obtain very fine bubbles for flotation is the dissolved air process. Wastewater is subjected to pressure in the presence of air, causing some air to dissolve. A flocculating agent, sometimes with a foaming agent, is injected into the liquid body. Upon sudden release of the pressure, the dissolved air comes out of solution to form a fairly uniform dispersion of fine air bubbles which float the floc. The effect is similar to the release of CO.sub.2 from carbonated beverages.
The low solubility of air in water obviously limits the amount of froth that can be generated by the dissolved air process, unless several cycles of pressurizing are used. In general, the cost of power and special equipment required, together with the mechanical difficulty in adaptation to continuous, high volume processing has limited the use of the dissolved air method.
Froth flotation has been applied to a variety of separation problems. Some soluble polluting species can be precipitated by suitable reaction and the precipitate subsequently removed by froth flotation. Obviously, by proper choice of agents, a mixture of species can be fractionated, separating only one specie in each flotation step.